I realize this is an old post, but I am still installing Ubuntu Server 14.04 and have found this issue as well. As far as I can tell it is from booting with EFI/UEFI media, as Rod Smith has stated.
I ran the install using a USB jump drive with the Ubuntu Server 14.04.4 ISO and the intention of installing in BIOS mode. In the BIOS menu I selected to boot from USB. The install ran normally and I created the partitions as desired (using 500mb ext2 in raid1 for the /boot partition) but I could not change the boot flag. Also, the partitioner did not ask for primary/logical partitions. The installer ran until the GRUB install, then failed. I attempted this multiple times with install variations with the same result.
Next I ran the install from a CD with the same ISO. I set up the partitions the same way and was able to set the bootable flag to on and also select primary/logical. The install then worked as expected and the GRUB install worked with complaint.
I am still new to installing and have not figured out all the details of BIOS/UEFI, but as far as I can tell this is where the issue lies.